Display element comprising phosphor and metal-insulator-metal bistable device



June 25,1968 J G BSMMONS ET AL 3,390,295

DISPLAY ELEMENT COMPRISING PHOSPHOR AND METAL-INSULAIOR-METAL BISTABLE DEVICE Filed April 15, 1967 Inventors JOH G. a. S/MMONS RUDOLPH R. VEROERBER Attorney United States Patent Oflice Patented June 25, 1968 3,390,295 DISPLAY ELEMENT COMPRISING PHOSPHOR AND METAL-lNSULATOR-METAL BlSTABLE DEVICE John George Bernard Simmons and Rudolph Richard Verderber, Harlow, England, assignors to International Standard Electric (Jorporation, New York, N.Y., a corporation of Delaware Filed Apr. 13, 1967, Ser. No. 630,697 Claims priority, application Great Britain, May 4, 1966, 19,712/66 7 Claims. (Cl. 313-408) ABSTRACT OF THE DISCLOSURE A solid state display element including a metal-insulator-metal non-volatile bistable device having a phosphor layer on one of the exposed metal surfaces. A transparent electrode overlies the exposed surface of the phosphor layer. Potentials may be applied between the metal layers to place the bistable diode in a selected one of two memory states. A potential subsequently applied between the outer metal layer and the transparent electrode causes electron emission from the diode element to excite the phosphor material, which radiates light observable through the transparent electrode when the diode is in a particular one of its two memory states.

Background the invention This invention relates to solid state display elements, and in particular to such elements which incorporate memory capabilities.

Bistable two terminal devices of metal-insulator-metal construction which exhibit a non-volatile memory capability are known in the art. In particular, devices are known which may be interrogated by applying a predetermined voltage between the device electrodes; electron emission through one of the electrodes is observed when the device is in a selected one of its states. While for the sake of clarity and simplicity such devices are described herein as being bistable, it should be understood that under suitable operating conditions multistable operation may be obtained; in this case any two sufficiently distinguishable states may be employed according to our invention, as long as significant electron emission is observed in only one of the two selected states.

The basic theory, construction and operation of the bistable metal-insulator-metal structure utilized in the manufaoture of devices according to our invention is describe-d in the following references: Negative Resistance and Electron Emission in Thin Insulating Sandwiches by Rudolph A. Cola, John G. Simmons, and R. R. Verderber, published in the Proceedings of the National Aerospace Electronics Conference, Dayton, Ohio, May 11, 1964.

Summary According to the invention a solid state display device includes (i) "a non-volatile bistable device capable of emitting electrons when voltage biased in one of its stable states and (ii) an electron sensitive phosphor adjacent to the electron source, so that when the device is voltage biased in one stable state the phosphor is luminous but when the device is biased in the other stable state the phosphor is not luminous.

According to one embodiment of the invention the solid state display device includes a non-volatile bistable device capable of emit-ting electrons when voltage biased in one of the stable states only, the device comprising a film of insulating material between two metal electrode films one of which has an aperture, a layer of electron sensitive phosphor material adjacent the aperture electrode and in contact with the insulating material through the aperture, and a transparent electrode on the surface of the phosphor material remote from the aperture.

In the drawing:

FIG. 1 is a diagrammatic cross section of the device; and

FIG. 2 is a voltage-current characteristic through the first three layers forming the electron source and memory diode.

Detailed description The display element illustrated in FIG. 1 is constructed as follows. An inert substrate, such as a sheet of glass 1, is masked and has deposited thereon a layer of gold '2 approximately 2,000 Angstroms thick. Part of the layer 2 is masked to provide an electrode contact area and then a layer 3 of silicon oxide about 1,000 Angstroms thick is deposited over the original gold layer 2. The thickness of the silicon oxide layer is not critical and emitting devices have been made using silicon oxide films of thicknesses ranging from 200 Angstroms to 2000 Angstroms. The thickness of the silicon oxide layer is predetermined by thermally evaporating to completion, in a vacuum, a weighed amount of silicon oxide from a molybdenum boat or by monitoring the thickness during deposition using the quartz crystal technique, the average rate of deposition of the layer being 4 A./.sec. Then a second electrode layer 4 is deposited on the insulating layer 3. This layer 4 is aluminum and is about 2,000 Angstroms thick. Prior to depositing the layer 4 the insulating layer 3 is masked so that when the layer 4 is deposited an aperture S will be left in the middle of the insulating layer 3. Next a layer 6 of a phosphor material such as zinc sulphide, approximately 2,000 A-ngstroms thick, is deposited on top of the second electrode layer 4. The layer of phospho-r material fills the aperture 5 and is in contact with the insulating layer 3. Finally a transparent electrode layer 7 is deposited on top of the phosphor material 6. The transparent electrode layer 7 is conveniently a film of chromium deposited on top of a film of aluminum, the total thickness being of the order of 300 Angstroms.

The device is made without breaking vacuum and all the layers are deposited at a pressure of 10 torr. After fabrication the device is subjected to an initial forming process, because until it has been formed it will not exhibit the bistable characteristics which make it a useful device. The device is formed in a vacuum of 10 torr at \ambient temperature by applying 10 volts across the device with the gold electrode biased positively. Accumulated data suggests that the forming process is due to injected positive ions, emanating from the gold electrode.

The first three layers 2, 3 and 4 comprise a metal-insulater-metal diode which when suitably driven has nonvolatile memory capabilities. FIG. 2 illustnates the currentvoltage characteristic of such a diode. The curve ODCBA shows the DC. characteristic of the device; the region DCBA is termed the negative resistance region. The region OD of the curve is frequency-independent, that is, that portion of the curve is always regeneratd no matter what the frequency, providing it does not exceed the resistance/capacitance time constant of the device. However, if the curve is traced out to :say point B, and the voltage removmi quickly, i.e. a trailing edge shorter than about 10 milliseconds, and the voltage reapplied but of magnitude less than 01 volts, the curve OB will be traced out and not that portion of the curve OD. This memory is non-volatile, i.e. it gives non-destructive read-out, providing the voltage d is not exceeded. If the voltage d is exceeded then the characteristic switches back to the state OD. Similar results are true for any voltage exceeding d; as a further example, if the voltage is removed quickly $3 from point A on the curve, the memory will be the curve A.

It appears that electrolytic introduction of gold monovalent ions into a thin film insulator results in a change in the electrical characteristics of the insulator. It is postulated that the ions introduce a broad impurity band within the insulator and that the conduction electrons travel through the band by direct tunneling between sites provided by the ions. In this manner the observed thermal and isothermal D.C. V-I characteristics, including the negative resistance effect can be explained.

In order to explain the memory effects it is necessary to introduce the concept of charge (electron) storage or trapping within the insulator when the applied voltage exceeds d. The erasure of a memory state is postulated to be due to the release of the trapped charge.

The behavior of the device is such that electrons will not be emitted if the device remains in the stable state indicated at D, i.e. it is in a low resistance state and a relatively large current can flow through the diode. If the diode however is switched to its other stable state A, i.e. the high resistance state, electrons are emitted from the layer 3. These electrons emitted from the layer 3 can be used to energise the phosphor layer 6 since the latter is in contact with the electron source 3. To do this a suitable positive potential is applied to the anode layer 7 and an electric field established within the phosphor material. The electrons emitted from the layer 3 will be accelerated in the phosphor and will cause the phosphor to be excited and emit visible radiation that can be seen through the transparent electrode layer 7.

If the diode formed by the layers 2, 3 and 4 is switched to the high resistance point D of the curve, even if a potential is applied across the phosphor material the latter will not be luminescent since there are no electrons being emitted from the layer 3. Thus the device may be permanently biased across the phosphor material and it is only necessary to switch the memory diode from one state to the other to cause the device to be luminescent when required.

Several such display elements can be formed into a matrix to provide a pattern-display screen. To display a desired pattern the necessary elements are switched to the emitting state, i.e. are switched to point A on the characteristic, and then by applying an illuminating potential to all the phosphor layers only the elements in the emitting .state will cause the phosphor to be excited and the desired pattern will be displayed. The state can be easily switched and the patterns displayed can be changed quite rapidly.

The width of the electrodes which determines the device area can be varied depending upon the maximum current desired. It is possible to make the device area as small as desired with the present state of the art techniques with no change of behavior. The aperture 5 in the layer 4 should preferably be of the order of 0.001 inch across, or less, as if made too large the resolution will be degraded. The phosphor layer 6, zinc sulphide, is a low energy phosphor which can be made to glow by relatively low energy electrons. For a device such as that shown in FIG. 1 no special encapsulation is needed and the device can operate in a normal (air) atmosphere.

Alternative insulating materials which are suitable for the layer 3 are magnesium fluoride and cadmium sulphide.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

We claim:

1. A solid state display device including a non-volatil bistable device capable of emitting electrons when voltag biased in one of the stable states only, said device comprising a layer of insulating material of electron tunneling thickness between two metallic electrode films, one of said films having an aperture therein, a layer of electronsensitive phosphor material adjacent said apertured electrode film and in contact with said insulating layer through said aperture, and a transparent electrode on the surface of said phosphor layer remote from said aperture.

2. A device according to claim 1, wherein said insulating material is selected from the group consisting of silicon oxide, magnesium fluoride and cadmium sulphide.

3. A device according to claim 2, wherein the thickness of the portion of said insulating layer is on the order of 1000 Angstroms.

4. A device according to claim 2 in which one of the electrode layers adjacent the insulating layer comprises a thin film of gold.

5. A device acconding to claim 4 in which the other electrode layer adjacent the insulating layer comprises a thin film of aluminum.

6. A device according to claim 2 in which the phosphor material is zinc sulphide.

7. A device according to claim 6 in which the transparent electrode layer comprises a thin film of chromium deposited on a thin film of aluminum.

References Cited UNITED STATES PATENTS 2,931,915 4/1960 Jay 313-108 3,010,025 11/1961 Bramley et al. 313l08 3,274,024 9/1966 Hill et al. 313l08 ROBERT SEGAL, Primary Examiner. 

